To design, develop, certificate and produce a new top level civil or military aircraft is a complex undertaking. It is very tempting (and pretty much normal practice) for the marketing and
PR side to assume all will go according to the original plan and so ‘oversell’ the delivery timescales and costs.
At least let us be glad when the engineers have the final say re critical issues as seems to be the case with the 787 and unlike one past Shuttle launch for example.